Organic semiconductors are applied to not only electrophotographic photoreceptors but also electronic devices including an electroluminescence element (Non-Patent Document 1), thin-film transistor (Non-Patent Document 2), a solar battery (Non-Patent Document 3), and the like. Moreover, it has been considered that organic semiconductors can be applied to electronic paper, utilizing its flexibility (Non-Patent Document 4).
From the point of view of producing practical devices, it is desired that thin films can be produced by solution processing, which can be conducted at low cost. However, currently-available organic semiconductors used in electroluminescence elements and field-effect transistors are required to be produced by vacuum processing in many cases.
Vacuum-deposited films have been used in organic thin-film solar batteries for a long time. In order to solve this concern, a fullerene derivative including a conjugate polymer or a substituent introduced thereinto, capable of being formed into a thin film by solution processing, has been used since the development of a bulk hetero junction-type organic thin-film solar battery by N. S. Sariciftci in 1993 (Non-Patent Document 5). However, an aggregation structure of a polymer or a conjugate polymer is an amorphous structure, so that carrier mobility is low.
On the other hand, a liquid crystalline semiconductor has received attention in recent years as a novel organic semiconductor material which can be formed into a thin film by solution processing and has high electron mobility (carrier mobility) (Non-Patent Document 6). In a higher-order liquid crystal phase of a liquid crystal material having an aromatic ring in a core region thereof, a molecular aggregation structure similar to a molecular crystal is formed, so that high-speed electron conduction as in the molecular crystal can be achieved. Moreover, the liquid crystal material generally has a long chain of an alkyl group, so that solubility in an organic solvent is high. Thus, the liquid crystal material can be formed into a film by solution processing. In addition, in the liquid crystal phase, flexibility and flowability based on the degree of freedom of molecular motion are exerted, so that the liquid crystal material is characterized in that formation of grain boundary, which is a problem in polycrystal, thin film is suppressed, and a high quality semiconductor thin film having high carrier mobility can be produced easily. The inventors of the present invention have succeeded in producing a field-effect transistor by solution processing using a liquid crystalline semiconductor (Non-Patent Documents 7 to 8 and Patent Documents 1 to 3). The inventors of the present invention further found that even if a 3% strain is applied to a field-effect transistor produced on a polymer substrate by solution processing using a liquid crystalline semiconductor, the properties of the field-effect transistor are not at all changed (Non-Patent Document 9, Patent Document 3).
As a liquid crystalline semiconductor having p-type (also referred to as “p type”) conductivity, many liquid crystalline semiconductors such as a triphenylene derivative (Non-Patent Document 10), a phthalocyanine derivative (Non-Patent Document 11), a hexabenzocoronene derivative (Non-Patent Document 12), and an oligothiophene derivative (Non-Patent Document 13) are known.
As an n-type (also referred to as “n type”) liquid crystalline semiconductor, liquid crystalline fullerene having an alkyl group introduced thereinto has been reported, for example, and electron transport has been found therein (Non-Patent Document 14).
A perylene tetracarboxylic acid derivative long has been known as an n-type semiconductor. That is, first, a vacuum-deposited film of perylene tetracarboxylic acid bisimide has been studied for a solar battery (Non-Patent Document 15) or a field-effect transistor (Non-Patent Document 16). Further, it has been found that a perylene tetracarboxylic acid bisimide derivative having a plurality of alkyl chains introduced thereinto exhibits a liquid crystal phase (Non-Patent Document 17). Furthermore it has been reported that a perylene tetracarboxylic acid imide derivative having a gallic acid ether portion has high electron mobility by space charge limited current measurement (Non-Patent Document 18). Another perylene tetracarboxylic acid imide derivative exhibits a liquid crystal phase at around room temperature, and electron mobility at room temperature has been measured by a Time-of-Flight (TOF) method (Non-Patent Document 19). In addition, an n-type perylene tetracarboxylic acid derivative which exhibits a liquid crystal phase at room temperature has been synthesized (Non-Patent Document 20). Moreover, it has been studied that a substituent is introduced into an aromatic ring portion in order to improve solubility of a perylene tetracarboxylic acid derivative in an organic solvent (Non-Patent Document 21).